Christopher Nicola is the cave diver who uncovered evidence of the Stermers and Wexlers' occupation of the cave system at the war's outset.

Magnolia Pictures

Christopher Nicola, the avid spelunker who introduces No Place on Earth, has an appetite for the dramatic.

"Every cave I enter has a secret," he intones, as the documentary cuts between Nicola's New York City home and his progress through tight underground passages.

In 1993, while exploring a 77-mile-long gypsum cave in Ukraine, Nicola found a secret that had nothing to do with geology: a chamber where 20th-century people had clearly lived for some time. The locals Nicola asked about the discovery had little to say. "Maybe some Jews" had hidden there, he was told.

Good guess, although "repressed memory" might be more accurate. Nearly 30 members of two families, the Stermers and the Wexlers, spent 511 days in the cave when World War II began, in an area that was then part of Poland. They had made plans to emigrate to Canada, but Hitler invaded before their ship could sail.

Himself of Eastern European (though not Jewish) heritage, Nicola searched for more information for a decade. Eventually, he located some of the survivors — as nearby as the Bronx. (Others live in Montreal.) Many of them had kept their ordeal private, although the story was not entirely hidden.

Matriarch Esther Stermer, one of the movie's off-screen heroes, published a 1975 memoir, We Fought to Survive. But only 500 copies were printed, and most went to Jewish or Holocaust-oriented institutions.

Janet Tobias' documentary begins with Nicola and concludes with a 2010 trip back to the site, during which four survivors were accompanied by two grandchildren. In between, the movie uses episodes in which actors play the cave dwellers. (And occasionally their nemeses).

The two families were well-to-do, and were able to purchase some supplies for their first hideout in a smaller, more accessible cave. A few of them collected and sold scrap metal to buy food. Then German soldiers raided, arresting several of the grotto residents. Most survived that incident, which nonetheless has a ghoulish coda.

The survivors moved with few provisions to a second cave, which was deeper and more remote. Unlike the first, it had an internal water source, but getting food there was more complicated, and forays outside the tunnels were always dangerous.

Inevitably, No Place on Earth looks and feels a bit like In Darkness, Agnieszka Holland's real-life 2011 drama about a group of Jews who found refuge in the sewers of a then-Polish, now-Ukrainian city. Like Holland, Tobias emphasizes the darkness. She even gives a subterranean look to the direct interviews, placing the survivors before black backdrops illuminated by a single light. Among those who testify are Sonia and Sima Dodyk, who were little girls when their families went underground.

Although the story is told with narration rather than dialogue, Tobias relies too much on reconstruction. A more inventive melding of documentary and docudrama would have benefited the film, whose most moving scenes all involve real members of the families. A bit more historical and geographic context would also be useful.

Still, it's an unforgettable story. It's also one that remains raw, as is demonstrated not just by the area residents' reluctance to discuss it with Nicola 20 years ago. The movie's credits reveal that the reconstructed scenes were shot elsewhere in Eastern Europe — not in the region where caves preserved just a few of the millions marked for extermination.